Self propelled losing power

Years ago I got a Yard Machines front wheel drive self-propelled walk behind because I got hilly areas to mow and need the assist. Recently I noticed the front wheel drive is very weak going up hills and the wheels wont even turn. They turn on flat ground or small slopes. Is this the belt or something else? How to I diagnose it?
 
Is there a model number tag and a serial number? That would help us look up the parts diagram and possible service information
 
I noticed today when I lift the front wheels off the ground to get it spinning it will go forward normally after I set them back down but won't do it while the weight is on the front from a start going up a steep incline. It used to do burnouts. Also, the cable has about an inch of slack when I pull the lever. Seems like that's about what it's always been but maybe not.
Model# 12A-A13K729 Serial# 1E013KC2312
 
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